The present invention relates generally to devices for spraying finely dispersed liquids, and more particularly to the use of a disposable cartridge that is compatible with a handheld electrohydrodynamic (EHD) spray device.
Spraying using EHD technology (also referred to as electric field effect technology (EFET)) is a process where fluids or other bulk solutions are dispensed through electrically-charged nozzles. In an EHD spray nozzle, the material to be sprayed flows through a region of high electric field strength made possible by the application of a high voltage to the nozzles and associated nozzle geometry. The high voltage causes the fluid material to acquire an electric charge; the electric field present at the nozzle tips applies a pole to the fluid; the poled fluid charge induces a force that acts in opposition to the surface tension of the material. This surface charge causes the formation of at least one ligament of thin jet of material, causing comminution of the fluid into fine droplets.
One advantage of the EHD process is that high fluid forcing pressures are not required, thereby reducing high-velocity fluid movement and concomitant levels of noise associated with fluid dispersal. As the fluid exits the nozzle, the repelling forces of the surface charge balance against the surface tension of the material, causing the formation of a conical spray pattern (often referred to as a Taylor cone). The tip of the cone has the greatest concentration of charge, and, at that point, the electrical forces overcome the surface tension, generating the thin jet of material that breaks up into charged droplets of generally uniform size. These charged droplets are then readily attracted to the target, adhering readily to it. As portions of the target become coated with the material, the relative electrostatic potential between coated sections and uncoated sections causes subsequent application of the charged material to be preferentially attracted to an uncoated portion of the target, thereby promoting more uniform coverage. The charged nature of the droplets is further beneficial in that their like charge tends to force them to avoid agglomeration. Soon after being deposited on the target, the material loses its charge, leaving an electrically-neutral end product.
EHD technology is a useful way to overcome many of the limitations inherent in other spray application devices, as uneven application, repeated squeezing and releasing of a fluid-dispensing trigger, waste of fluid product and inadvertent exposure of the operator to the fluid material can be reduced or outright avoided. One example of where EHD technology is beneficial is in animal care products, where pesticides and related therapeutic products can be applied easily, accurately and with minimal inconvenience to the operator and the animal being treated. As well, EHD technology may be extended to other uses, including the application of cosmetics, personal care materials and medicaments to animals and humans, as well as the dispensing of fluids for other uses.
One way to further improve the operability of EHD spraying devices is to have the fluid being dispensed be provided in disposable cartridges. Once the product is dispensed, the cartridge can be thrown away and replaced by a new one. This is beneficial in situations where prolonged or excessive exposure to the fluid being dispensed is undesirable, such as with pesticides or other materials used to treat horses and other domesticated animals.
Within the cartridge art are containers in which a generally cylindrical-shaped piston is driven along the length of a complementary-shaped inner wall of the cartridge upon rotation of a lead screw. The lead screw is threaded through the piston and extends into the fluid chamber of the cartridge, and is sometimes referred to as a “wetted” lead screw. Fluid disposed downstream of the piston is forced through an outlet in response to the increasing pressure within the cartridge by piston movement in the downstream direction.
Unfortunately, the above-mentioned cartridge is prone to leakage, especially in regions between the outer periphery of the piston and the inner wall of the cartridge, as well as the threaded space between the screw and the piston. In an application where a cartridge of this type may be used in a device with electronics, the fluid can potentially leak into regions where electronic and other liquid-intolerant componentry resides. As well, when the cartridge or device is being stored during long periods of time during shipping, storage, display or between uses, an unacceptable quantity of fluid may be lost. This problem is particularly acute in situations where the liquid is expensive or hazardous, such as a pesticide, herbicide, flammable materials or the like.
What is desired is a leak-free cartridge, and more desirably, a leak-free disposable cartridge that can be used with an EHD device that is inexpensive to manufacture and easy to dispose of once the contents are dispensed.